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Previously collected and new water-quality data from shallow wells (screened interval less than 30 meters below the land surface) in predominantly agricultural areas of the New Jersey Coastal Plain were used to determine the relation of nitrate concentrations in shallow ground water to various hydrogeologic and land-use factors in the study area. Information on land use, well construction, hydrogeology, and water quality were used to predict the conditions under which concentrations of nitrate as nitrogen in water from domestic wells in predominantly agricultural areas are most likely to be equal to or larger than the U.S. Environmental Protection Agency maximum contaminant level (MCL) of 10 milligrams per liter. Results of the analyses of water-quality samples collected during 1980-89 from 230 shallow wells in the outcrop areas of the Kirkwood-Cohansey and Potomac-Raritan-Magothy aquifer systems were used to evaluate the regional effects of land use on shallow-ground-water quality. Results of statistical analysis indicate that concentrations of nitrate in shallow ground water are significantly different (p = 0.001) in agricultural areas than in undeveloped areas in both aquifer systems. Concentrations of nitrate nitrogen exceeded the MCL in water from more than 33 percent of the 60 shallow wells in agricultural areas. Concentrations of nitrate in water from shallow wells in agricultural areas increased as the percentage of agricultural land within an 800-meter-radius buffer zone of the wellhead increased (r = 0.81). Concentrations of nitrate in water from domestic wells in agricultural areas were similar (p = 0.23) to those concentrations in water from irrigation wells. These results indicate that most of the nitrate in water from domestic wells in agricultural areas results from agricultural practices rather than other sources, such as septic systems. Water-quality samples collected from 12 shallow domestic wells in agricultural areas screened in the outcrop areas of the Kirkwood-Cohansey and Potomac-Raritan-Magothy aquifer systems were used to evaluate the local effects of hydrogeologic conditions and land-use activities on shallow-ground-water quality. Concentrations of water-quality constituents in these wells were similar among four sampling events over a 1-year span. The concentration of nitrate in water from 6 of the 12 wells exceeded the MCL. Concentrations of nitrate greater than the MCL are associated with: values of specific conductance greater than 200 microsiemens per centimeter at 25 degrees Celsius, a screened interval whose top is less than 20 meters below land surface, concentrations of dissolved oxygen greater than 6 milligrams per liter, presence of pesticides in the ground water, a distance of less than 250 meters between the wellhead and the surfacewater divide, and presence of livestock near the wellhead. Ratios of stable isotopes of nitrogen in the water samples indicate that the source of nitrate in the ground water was predominantly chemical fertilizers rather than livestock wastes or effluent from septic systems. INTRODUCTION Contamination of ground water by agricultural chemicals, especially nitrates, is a major concern of environmental and regulatory agencies throughout the nation. Nitrate is soluble in water and is transported from its source along local and regional ground-water flow paths. Although nitrate is relatively nontoxic, in the intestines of infants it can be reduced by bacteria to nitrite and may result in the disease methemoglobinemia. Infant mortality from this disease is rare where concentrations of nitrate as nitrogen (as N) in drinking water are less than 10 mg/L; however, the incidence of mortality increases with increasing concentration (Walton, 1951). The U.S. Environmental Protection Agency (USEPA) has set a maximum contaminant level (MCL) of 10 mg/L for nitrate (as N) in water (U.S. Environmental Protection Agency, 1991). An MCL is the maximum permissible level of a contaminant in water that is delivered to any user of a public water system. As a result of earlier studies that showed that concentrations of nitrate in ground water in agricultural areas commonly exceeded the MCL, a project was initiated by the U.S. Geological Survey (USGS) in cooperation with the New Jersey Department of Environmental Protection and Energy (NJDEPE) as part of the New Jersey Safe Drinking Water Program. Purpose and Scope The purpose of this report is to (1) document the seasonal variability of ground-water-quality constituents in shallow domestic wells in agricultural areas of the Coastal Plain of New Jersey; (2) describe the use of hydrogeologic, land-use, well-construction, and water-quality characteristics and constituents as indicators to predict whether the concentration of nitrate in water from a shallow domestic well in those areas will exceed the MCL; and (3) identify the predominant source(s) of nitrate in water from shallow domestic wells in agricultural areas. Previous Investigations In 1984, as part of its Toxic Substances Hydrology Program, the USGS began to evaluate the degradation of ground-water quality as a result of human activities. These studies were designed to test statistically the hypothesis that shallow-ground-water quality is related to human activities expressed as land use (Helsel and Ragone, 1984). Preliminary results from study areas in New York, New Jersey, Connecticut, Florida, Nebraska, and Colorado indicate that regional ground-water quality has been affected by human activities (Cain and others, 1989, p. 230). The relation between nitrate in shallow ground water and land use in the outcrop area of the Potomac-Raritan-Magothy aquifer system in the New Jersey Coastal Plain was assessed by Barton and others (1987) and Kish and others (1988). Louis and \fowinkel (1989) evaluated the presence of agricultural chemicals in ground water in the outcrop areas of the Potomac-Raritan-Magothy aquifer system and the Kirkwood-Cohansey aquifer systems. Vowinkel (1991) reported that the relation between land use and nitrate in shallow ground water was similar between the two Coastal Plain aquifer systems; concentrations of nitrate in shallow ground water were significantly different in agricultural areas than in undeveloped areas. Water from more than 33 percent of the shallow wells in predominantly agricultural areas contained concentrations of nitrate exceeding the MCL. Results of the USEPA's National Pesticide Survey indicate that concentrations of nitrate in ground water were equal to or greater than the MCL in 1.2 percent of the community and 2.4 percent of the rural wells nationwide. A statistical analysis of the USEPA data indicated that well depth was the only consistent predictor of nitrate concentration in ground water in community and rural wells (U.S. Environmental Protection Agency, 1992); however, because the hydrogeologic variables used in the model, such as the presence of confining units, were determined at the county scale rather than at a site or hydrogeologic-unit scale, the results of this study were limited. The USEPA findings indicate that further stratification of the data is needed to accurately test hypotheses concerning the associations among variables. Results of a study conducted in Mercer and Burlington Counties, New Jersey (Murphy, 1992) demonstrated that the concentrations of nitrate in water from shallow drinking-water wells (screened intervals less than 30 m below the land surface) were significantly different in agricultural and residential areas than in woodland areas. Concentrations of nitrate in shallow ground water were significantly different in wells less than 15m from a septic system than in wells farther than 15m from a septic system; this result indicates that effluent from septic systems may have a significant effect on nitrate concentrations in water from sampled wells. Acknowledgments The authors thank the homeowners and farmers who allowed access to their wells for collection of water samples. Their cooperation is greatly appreciated. DESCRIPTION OF THE STUDY AREA The Coastal Plain physiographic province covers about 10,900 km2 in southern New Jersey (fig. 1). The study area consists of a variety of hydrogeologic and land-use settings that can affect ground-water quality. Hydrooeologic Setting The Coastal Plain is underlain by a seaward-dipping wedge of unconsolidated sediments consisting of alternating layers of sand, silt, and clay of Cretaceous to Holocene age (fig. 2). The hydrogeologic framework of (Zapecza, 1989) and regional ground-water flow in (Martin, 1990) the New Jersey part of the Coastal Plain are well-documented. The Coastal Plain is separated from other physiographic provinces in New Jersey by the Fall Line, which extends from Raritan Bay in the northeast to Delaware Bay in the southwest. Two major aquifers, the Potomac-Raritan-Magothy aquifer system and the KirkwoodCohansey aquifer system, supply more than 90 percent of the freshwater used by the 3.2 million people living on the Coastal Plain in New Jersey. The Potomac-Raritan-Magothy aquifer system in the Cretaceous Potomac-Group, Raritan-Formation, and Magothy Formation underlies the entire Coastal Plain but crops out in a narrow 5to 16-km-wide strip of land that comprises about 1,000 km2 and extends from Raritan Bay to Delaware Bay. The outcrop area of the Kirkwood-Cohansey aquifer system in the Miocene Kirkwood Formation and the Cohansey Sand exceeds 7,500 km2. Hydrogeologic conditions in the Coastal Plain are such that shallow ground water is vulnerable to contamination from point and nonpoint sources. The land surface generally is flat; relief rarely is greater than 30 m in a drainage basin. Agricultural areas typically are in the upland parts of the drainage basin, where the soils are well-drained. Soils in agricultural areas in the northern part of the outcrop area of the Potomac-Raritan-Magothy aquifer system consist predominantly of san

Indicators of the sources and distribution of nitrate in water from shallow domestic wells in agricultural areas of the New Jersey Coastal Plain